Ultra-thin plasma panel radiation detector

ABSTRACT

An ultra-thin radiation detector includes a radiation detector gas chamber having at least one ultra-thin chamber window and an ultra-thin first substrate contained within the gas chamber. The detector further includes a second substrate generally parallel to and coupled to the first substrate and defining a gas gap between the first substrate and the second substrate. The detector further includes a discharge gas between the substrates and contained within the gas chamber, where the discharge gas is free to circulate within the gas chamber and between the first and second substrates at a given gas pressure. The detector further includes a first electrode coupled to one of the substrates and a second electrode electrically coupled to the first electrode. The detector further includes a first discharge event detector coupled to at least one of the electrodes for detecting a gas discharge counting event in the electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of U.S. patentapplication Ser. No. 15/404,713, filed on Jan. 12, 2017, which is acontinuation application of U.S. patent application Ser. No. 14/218,820,filed on Mar. 18, 2014, now U.S. Pat. No. 9,551,795, issued Jan. 24,2017, which claims priority to U.S. Provisional Patent Application Ser.No. 61/852,426, filed on Mar. 15, 2013. The specifications of each ofthese applications is herein incorporated by reference.

GOVERNMENT LICENSE RIGHTS

This invention was made with government support under a Small BusinessInnovation Research (“SBIR”) Assistance Agreement, Grant No.DE-SC0006204, awarded to Integrated Sensors, LLC, by the U.S. Departmentof Energy. The government has certain rights in the invention.

FIELD

One embodiment of the present invention is directed to the detection ofradiation. More particularly, one embodiment of the present invention isdirected to a plasma panel based detection of radiation.

BACKGROUND INFORMATION

Many useful applications, such as the detection of radioactive materialand computer-assisted tomography (“CAT”), rely on the detection ofphoton radiation, known as X-ray and/or gamma-ray radiation. Both ofthese types of high-energy photon radiation cause ionization and for thepurposes of this disclosure the two terms, X-ray and gamma-ray, are usedinterchangeably. In terms of the detection of such ionizing radiation,the spectral region of greatest interest for most of these applicationsgenerally falls between the energies of about 20 keV to 20 MeV. Otherapplications, including the detection of particle radiation from ionbeam accelerators/colliders, cosmic ray generated minimum ionizingparticles (“MIP”s), and neutrons from special nuclear materials (“SNM”)used in nuclear weapons (e.g., enriched uranium or plutonium-239), relyon the detection of ionizing particles that can be either atomic nuclei(e.g., alpha particles), or subatomic (e.g., neutrons, protons andmuons) in nature, and which can vary over a very broad energy range fromless than 1 KeV to well beyond 1 TeV.

In order to detect ionizing radiation in the above spectral range ofinterest, a number of known sensing devices are commonly used. One ofthe earliest known electronic devices is the ionization chamber.Detection of radiation in an ionization chamber, such as aGeiger-Mueller (“GM”) tube, is based upon electrical conductivityinduced in an inert gas (usually containing argon, neon or helium as themain component) as a consequence of ion-pair formation. One currentlywidely used type of ionizing-particle radiation detector is themicropattern gas detector. These devices have been under continuousdevelopment for many years in high energy and nuclear physics. Detectorssuch as the Microstrip Gas Chamber (“MSGC”), Gas Electron Multiplier(“GEM”) and Micromegas have many desirable properties as proportionalgas detectors, but are operationally limited to gains within theproportional region in the range of ˜10³ to 10⁶.

A new class of radiation detectors, known as plasma panel sensors(“PPS”), have been introduced within the past decade and are derivedfrom the technologies used in producing plasma display panels (“PDP”)for television. Compared to conventional gaseous detectors, thesedevices, which can have significantly higher gain, fast response andvery high position resolution, encompass some of the best features of GMtubes and conventional micropattern gas detectors. PPS based detectorsare inherently digital in nature and operate in the high gain,non-linear (i.e. non-proportional) Geiger mode region. This feature isunique relative to other known high quality radiation detectors that areproportional in nature and as such are confined to operation in thelinear region. As such, each cell or pixel in a plasma panel basedradiation detector can be thought of as generating a micro-Geiger typedischarge.

SUMMARY

One embodiment is an ultra-thin radiation detector that includes aradiation detector gas chamber having at least one ultra-thin chamberwindow and an ultra-thin first substrate contained within the gaschamber. The detector further includes a second substrate generallyparallel to and coupled to the first substrate and defining a gas gapbetween the first substrate and the second substrate. The detectorfurther includes a discharge gas between the first and second substratesand contained within the gas chamber, where the discharge gas is free tocirculate within the gas chamber and between the first and secondsubstrates at a given gas pressure. The detector further includes afirst electrode coupled to one of the substrates and a second electrodeelectrically coupled to the first electrode. The detector furtherincludes a first impedance coupled to the first electrode, a powersupply coupled to at least one of the electrodes, and a first dischargeevent detector coupled to at least one of the electrodes for detecting agas discharge counting event in the electrode. The detector furtherincludes a plurality of pixels defined by the electrodes, each pixelcapable of outputting a gas discharge counting event pulse uponinteraction with ionizing radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a columnar-discharge PPS having a two-electrodestructure in accordance with one embodiment.

FIG. 2 is a perspective view of a columnar-discharge ultra-thin PPS withan “open-cell” electrode structure having a two-electrode,double-substrate, columnar-discharge (“CD”) configuration in accordancewith one embodiment.

FIG. 3 illustrates a columnar-discharge PPS test panel (i.e., theorthogonal electrode structure in FIG. 1) after modifying a commercialtwo-electrode, DC-type, glass PDP in accordance with one embodiment.

FIG. 4 is a side view of an ultra-thin PPS detector contained within anultra-thin window gas chamber enclosure with foil windows in accordancewith one embodiment.

FIG. 5 is a side view of an ultra-thin PPS detector contained within anultra-thin window gas chamber enclosure with double foil windows inaccordance with one embodiment.

FIG. 6 is a side view of a vertical stack on internal ultra-thin PPSdetectors contained within an ultra-thin window gas chamber enclosurewith foil windows in accordance with one embodiment.

FIG. 7 illustrates the use of embodiments of the invention for medicalCT imaging.

FIG. 8 is a perspective view of an ultra-thin surface-discharge PPSpixel array in accordance with one embodiment.

FIG. 9 illustrates a fabricated alumina grid-support plate in accordancewith one embodiment.

FIG. 10 is a two-dimensional perspective drawing of a grid-support PPSstructure in accordance with embodiments of the invention.

FIG. 11 illustrates an example of a grid-support array formed withthick-film patterning in accordance with one embodiment.

FIGS. 12A-C illustrate an ultra-thin PPS with a second exhaust valve inaccordance with one embodiment and a surface-discharge electrodestructure in accordance with another embodiment.

FIG. 13 illustrates the bottom substrate of a PPS structure thatincorporates a cathode cell quench resistor located on the back side ofthe bottom substrate, with one resistor for each cell, in accordancewith one embodiment.

FIG. 14 is a perspective view of a grid-support PPS structure inaccordance with one embodiment.

FIG. 15 is a top view of a more detailed composite overlay view of thethree plate PPS structure of FIG. 14 in accordance with one embodiment.

FIG. 16 is a graphical illustration of volts vs. time in nanosecondsillustrating a characteristic PPS signal induced by a beta source inaccordance with embodiments of the invention.

FIG. 17 is a block diagram of pulse-counting type electrode circuitryfor detecting each gas discharge cell interaction and counting each suchinteraction as an individual pixel discharge event in accordance withone embodiment.

DETAILED DESCRIPTION

One embodiment is an ultra-thin plasma panel sensor (“PPS”) detectorfabricated using extremely thin, low mass substrates. Embodiments can beused for the detection, tracking, identification, position sensingand/or imaging of ionizing particles, ionizing particle beams or photonsgenerated by any means.

In general, the PPS in accordance with embodiments of the presentinvention differ from most other known gaseous micropattern particledetectors for at least the following reason:

Sparking and Gain: A recurrent problem with micropattern detectors whichoperate with gains of ˜10⁴ (and greater) is possible destructivesparking. The PPS is designed to typically be a higher gain, Geiger-modedevice and should be effectively immune to destructive sparking. Aninline current-limiting quench resistor associated with every PPS pixel,or pixel high-voltage cathode line, immediately drops the voltage atdischarge and terminates the current pulse.

Spatial Resolution: Mature photolithographic and ion milling techniquescan be used to deposit and pattern electrodes with micron-levelprecision and with cell pitches of less than 100 μm; currentmanufacturing capability already far exceeds this precision thusproviding a direct path to high resolution PPS devices.

Fast Response and High Rates: Signal development depends on gasavalanche and streamer formation over a narrow gap. These processes areintrinsically fast, on the order of nanoseconds or less, depending ongas gap geometry, gas mixture, etc. Rate capability is determined by thecell recovery time, which can be made shorter by the addition of asuitable gas quenching component as used in Geiger tubes. In tests usingcommercial PDPs with large capacitances, recovery times are on the orderof ˜10 μsec. With pixel densities from hundreds to thousands ofcells/cm², the hit rate capability can potentially reach ˜100 MHz/cm².

Cost, Scalability, Electronics: PPS detectors can benefit from similarfabrication processes, materials and associated mechanical andelectrical properties as large area PDPs. Current prices for 40-50 inchdiagonal PDPs are ˜$0.02 per cm². PPS readout electronics would besimilar to those used in other high channel density, two coordinatedetectors. The PPS high gain renders them intrinsically binary,generally obviating the need for an amplification stage and thussimplifying the front-end signal processing. Because of being fabricatedon glass or ceramic substrates, high density, high speed electrode tointegrated circuit interconnections can be achieved via low cost,chip-on-glass (“COG”) type technology, such as that used on PDP and LCDdisplays.

For many of the uses described herein, fabrication of PPS structures inaccordance with embodiments can utilize low cost materials andmanufacturing processes, especially those developed for the fabricationof high resolution, flat panel displays such as PDPs, LCDs and OLEDs.Radiation detectors that are based on a PDP are known, for example, asdisclosed in U.S. Pat. No. 7,332,726, and U.S. Pat. Pub. No.2010/0265078, the disclosure of each of which is hereby incorporated byreference. For example, PDPs are mass-produced as large area (e.g., 1-2meter diagonal) flat panel displays almost exclusively for HDTVtelevision applications. Such panels currently sell for about twoorders-of-magnitude less per unit area than the lowest costphotomultiplier tubes. However, the trend in industry is to buildthinner and thinner display devices, especially for portableapplications such as smart phones, tablets and laptop computers. Thelogical extension of this trend is the development of “flexible”high-resolution displays fabricated on flexible substrates such asflexible glass or plastics. Glass manufacturers have developed flexibleglass for roll-to-roll processing in thicknesses ranging from about 25to 200 microns. The new PPS configurations and embodiments describedherein take advantage of this trend towards ever thinner substrates vianew PPS device structures designed to achieve the lowest possible massby incorporating ultra-thin materials (e.g., glasses, ceramics, metalfoils and certain barrier coated plastic films), with some of thesematerials having a thickness in the range of 1 to 10 microns.

Embodiments of the present invention are directed to radiation detectorsbased on the plasma panel sensor for the purpose of realizing enhancedperformance with respect to the detection of ionizing radiation in theform of both photons and particles. Such devices can be used for thedetection, tracking, identification and/or imaging of ionizing photonsor particles generated by any means. The ionizing particles to bedetected by embodiments can include both neutral and lightly to heavilycharged particles, spanning the range from relatively low energies inthe range of keV to relativistic particles (e.g., hundreds of GeV andTeV). Some of the accelerator related applications for these detectorsinclude: the detection, monitoring and/or profiling of protons andheavier ions such as carbon and neon ions in the treatment of cancer byhadron particle beam therapy; the detection of somewhat higher energyparticles including protons for example for proton CT-imaging as analternative to conventional X-ray based CT-imaging; the detection,monitoring and profiling of very large ionizing particles generated inradioactive ion beam (“RIB”) accelerators; and the detection andtracking of subatomic ionizing particles such as muons generated atfacilities such as the Large Hadron Collider (“LHC”) at CERN. Otherapplications include the detection of particles or photons fromradioactive sources including the detection of neutral ionizingparticles such as neutrons. Additional applications include thedetection of: UV, (ultraviolet) and/or VUV (vacuum ultraviolet) and/orsoft X-ray photons generated by high temperature flames such as in gasor liquid or solid fuel based turbine engines used for generatingelectricity or in jet engines or by welding arcs or electron beams,etc., ionizing radiation including gamma-rays for applications inastronomy, ionizing radiation for oil and gas exploration andnondestructive testing, etc.

Embodiments include plasma panel radiation detectors that incorporatenovel structures based on the use of very low mass, thin and ultra-thinsubstrates to minimize interaction with the incident radiation by thenon-active device support materials in the radiation beam path. Thisrequirement is especially important for particle and photon beamtracking applications or other such applications that require hightransparency and minimal scattering of the incident particles or beam bythe radiation detector itself. For example, medical radiation detectorsthat are positioned between the radiation source and the patient shouldgenerally be as transparent as possible with minimal scattering. Suchscattering needlessly exposes the patient to additional radiation thatserves no useful purpose; in fact the resulting radiation attenuationand scattering degrades the incident beam quality and typically also theimage quality if the beam is used for diagnostic imaging purposes.

As disclosed, the term “thin” in one embodiment describes substrateswith a thickness between approximately 0.65 to 1.3 mm, while the term“ultra-thin” is used to describe substrates with a thickness less than0.65 mm which includes “flexible substrates” that typically define athickness range of less than or equal to about 0.25 mm. However, “thin”may encompass “ultra-thin”, and vice versa. “Low mass” may encompassboth thin and ultra-thin. This description of thickness ranges for thin,ultra-thin and flexible substrates (flexible being a subcategory ofultra-thin) is somewhat arbitrary and material dependent, and as suchshould be considered as only an approximate guideline. For example, 0.2mm thickness glass is described by Corning Incorporated as “flexible”glass and is sold in large diameter rolls and has been developed forroll-to-roll processing. Further, plastics and metals in this thicknessare definitely flexible, but ceramics in a thickness of 0.2 mm aregenerally considered as being rigid and typically handled as such. Inthe case of metals and coated polymers, two subcategories under theultra-thin or flexible description are metal “foils” and polymer“films”, which typically describe flexible metal sheets in thicknessesof about 0.1 mm and thinner, and flexible polymers in thicknesses ofabout 0.3 mm or thinner.

The PPS device in accordance with one embodiment includes commoncomponents with plasma display panels (“PDP”s) developed for TVapplications. PDPs are the core component of flat panel plasmatelevisions. Their design and production is supported by an extensiveand experienced industrial base with approximately four decades ofdevelopment. A PDP television comprises millions of cells per squaremeter, each of which, when provided with a signal pulse, can initiateand sustain a plasma discharge. Configured as a PPS radiation detector,each cell is typically biased to discharge in the Geiger mode whenfree-electrons are generated or injected into the gas by an externalsource of incident ionizing radiation entering or passing through theactive pixel region. The PPS thus functions as a position-sensitive,highly integrated array of parallel pixel-sensor-elements or cells, eachindependently capable of detecting free-electrons generated in ordeposited into the cell. The discharge is self-limiting and can beeffectively quenched by a localized resistance associated with eachpixel site, or, in cases of low incident radiation intensity, alocalized resistance on each line. The device cell structure, gasmixture and circuitry are designed to isolate the gas discharge so thatcell crosstalk, discharge regeneration and discharge spreading can beminimized if not eliminated, thereby enhancing the device capability asa high resolution, position-sensitive radiation detector. The devicecircuitry can detect when and where a gas discharge counting event pulseoccurs, counting each such gas discharge pulse as an individual eventand having an approximately equal value. For particle counting, imagingand radiation dosimetry applications, the PPS can provide a highlylinear, accurate, and quantitative digital response to an amount ofincident radiation based on either a sum of local individual counts fora restricted pixel area of illumination or on an all-inclusive totalcount of individual events across the entire device area of interest.Thus an amount of detected radiation is based on a total count ofindividual events. This linear response to the incident radiationintensity or flux is based on the fast, inherently-digital, particlecounting nature of the PPS device which operates in the high gain,non-linear Geiger-mode response region. In general, the PPS inaccordance with embodiments have a linear digital counting response, yetoperate in the non-linear Geiger-mode response region

PPS devices in accordance with embodiments detect charged particlesprimarily by direct gas ionization. FIG. 1 illustrates acolumnar-discharge PPS 100 having a two-electrode structure inaccordance with one embodiment. PPS 100 is an open structure andincludes a plurality of electrode crossing points or paired proximitypixel locations (e.g., crossing point 110) separated by a gas gap. PPS100 further includes a hermetic glass seal 105. Not shown is theinternal pixel dielectric structure, or the internalgas-gap/discharge-gap panel spacers. PPS 100 has an open-cell orthogonalX-Y electrode structure. “Open-cell” means that there is no ribenclosure surrounding each cell, which makes these devices considerablyeasier to fabricate than PDPs for TV applications.

FIG. 2 is a perspective view of a columnar-discharge ultra-thin PPS 200with an “open-cell” electrode structure having a two-electrode,double-substrate, columnar-discharge (CD) configuration in accordancewith one embodiment. PPS 200 includes a front substrate 201, a backsubstrate 205, X-electrodes 203 and Y-electrodes 206. PPS 200 furtherincludes quenching resistors 202 and a discharge gap 204. In otherembodiments the panels also include an “open”, thick-film, dielectricsurrounding window cell layer (not shown) deposited on top of theY-electrodes. The substrates can be glass, ceramic, fused silica,glass-ceramic, sapphire, etc. Flexible glass substrates that arecommercially available in thicknesses from 25 to 200 microns thick, andfused silica and ceramic substrates that are available at a minimumthickness of about 125 microns, can be used with some embodiments.

The discharge occurs in the volume defined by the intersection of thefront column electrodes 203 (e.g., high voltage-cathodes) and the backrow electrodes 206 (e.g., sense anodes) as shown in FIG. 2. With largegas-gap embodiments, the entire cell volume will be active and ion pairscreated anywhere in this gas volume should be drawn by the high fieldgradients to the nearest cell electrodes that define the localizeddischarge space and hence a 2-dimensional position location.

FIG. 3 illustrates a columnar-discharge PPS test panel 300 (i.e., theorthogonal electrode structure in FIG. 1) after modifying a commercialtwo-electrode, DC-type, glass PDP in accordance with one embodiment.Panel 300 in FIG. 3 is attached to a removable aluminum frame 301 formechanical integrity, which is fitted with a sealed, high-vacuum,shut-off valve 302 to allow multiple fills of different gas mixtures andpressures. The panel active cell area in this embodiment is 8.1×32.5 cmand was made with 2.3 mm thick soda-lime glass substrates. Theelectrode/cell pitch of the panel shown in FIG. 3 is 2.5 mm. Othertested devices have a 1.0 mm cell pitch. A readout electronics card (notshown) mounts on the horizontal anode lines and the signal is picked offusing a 50 ohm termination resistance. A high-voltage bus feeds thevertical cathode lines via a single quench resistance per line as shownif FIG. 2.

The thin and ultra-thin PPS embodiments disclosed herein can address anexpanded set of radiation detection and imaging applications inproviding higher performance in terms of higher efficiency, reducedscattering, and/or improved tracking and position resolution capability.In particular, panel 300 shown in FIG. 3 has 2.3 mm thick glass, whichcan cause significant scattering of low to medium energy incidentparticles, thus the need for much thinner substrate PPS devices forapplications that require minimal incident particle scattering. Becausethe boundary or distinction between “thin” and “ultra-thin” materialsoverlaps, and the numerical definitions provided above are somewhatarbitrary and material dependent, the two terms can be usedinterchangeably so that the term “ultra-thin” also can encompass “thin”devices, as defined above, and vice versa. Thus, like other PPS devices,the new ultra-thin configurations described herein inherit from PDPsseveral key attributes attractive for ionizing radiation detectors. Forexample, the basic PPS device can be fabricated from intrinsicallyrad-hard materials and sealed with a stable gas mixture, thus resultingin a device with extensive lifetime. However for those applicationsrequiring the lowest possible mass and the thinnest possibleconstruction, hermetic sealing may not be practical and periodic gasexchange via either a gas valve system such as shown in FIG. 3, or a gasflow-through system or a gas filled chamber enclosure as described belowmay be required. This is because many ultra-thin substrate materials arenot sufficiently impermeable to ambient gaseous diffusion into thedevice active gas discharge region over a long period of time. Forexample, at a thickness of 1 micron there is no substrate that issufficiently impermeable to gaseous diffusion over a time periodmeasured in years for making a stable hermetically-sealed PPS devicehaving a small gas volume. Similarly there is no known substratematerial with a thickness of a few microns that can be self-supportingover an unsupported area on the order of one or more square incheswithout noticeable surface distortion/bending under a pressuredifferential that can approach one atmosphere.

Embodiments provide solutions to the above described problems byenclosing the ultra-thin PPS within an ultra-thin single or multi-walledwindow enclosure having the same gas composition and pressure bothinside (i.e., internal) and outside (i.e., external) the PPS front andback substrate structure. In this manner, gaseous diffusion into the PPSwill not change the gas composition, nor would there be a pressuredifferential if the enclosure is properly designed as either a gas-flowsystem or a periodic gas exchange static system with the same gascomposition and gas pressure distributed between the wall windows andthroughout the enclosure as within the PPS. In general, the outer-mostwall windows facing the external ambient atmosphere would distort tosome extent due to the presumed pressure differential, but if properlydesigned by means of either minimizing the pressure differential or byusing a multi-walled window structure with the same pressure inside thechamber as between the window walls, the inner-most wall windows shouldbe able to remain almost perfectly flat. In general, in embodiments, theouter-most wall windows of the PPS enclosure/chamber is functionallysacrificial so as to maintain the planarity and uniformity of theinternal gas gap between the front and back ultra-thin PPS substrates.The ultra-thin wall window enclosure/chamber structure can employ, forexample, ultra-thin aluminum (e.g. 13-25 microns) or titanium foil(e.g., 6-13 microns) windows, or an even thinner high-strength alloyfoil such as cobalt-based Arnavar™ or Havar® foil, or perhaps even alower mass coated or metalized polymer film window. For example,double-sided metalized 1.5 micron thick PEN film (i.e., polyethylenenaphthalate) is commercially available, as is double-sided metalized 6micron thick BoPET (i.e., biaxially-oriented polyethylene terephthalatewhich is Mylar®), PEEK (polyether ether ketone), PEI (polyetherimide),etc. The use of metalized or otherwise coated polymer films as opposedto an uncoated polymer window is to minimize gas diffusion/permeationthrough the base polymer. This because coated polymer films are able toreduce gas diffusion and permeation by orders-of-magnitude compared tothe base polymer. The most common example of this is helium filledballoons which almost always use an aluminized barrier layer coated on apolymer base film (e.g., 0.001″ thick Mylar® or PET) to prevent rapiddiffusion of the helium out of the balloon.

FIG. 4 is a side view of an ultra-thin PPS detector 402 contained withinan ultra-thin window gas chamber enclosure 401 with foil windows 403 ontwo opposite walls in accordance with one embodiment. PPS 402 (i.e., an“internal” radiation detector) is not hermetically sealed, but insteadopen to the gas atmosphere of the chamber and at the same pressure asthe chamber 401 which can have an optional gas regulation systemattached to control the internal gas pressure. Gas input 405 and exitvalves 406 control the gas flow, which is maintained close to theambient pressure. A slightly negative chamber pressure is shown in FIG.4, but it could also be slightly positive.

FIG. 5 is a side view of an ultra-thin PPS detector 502 contained withinan ultra-thin window gas chamber enclosure 501 with double foil windows503 in accordance with one embodiment. PPS 502 is not hermeticallysealed, but instead open to the gas atmosphere within the chamber and atthe same pressure as the controlled chamber pressure. Gas input 505 andexit valves 506 control the gas flow, which is maintained close to theambient pressure. A slightly negative chamber pressure is shown in FIG.5, but it could also be slightly positive.

FIG. 6 is a side view of a vertical stack on internal ultra-thin PPSdetectors 602 contained within an ultra-thin window gas chamberenclosure 601 with foil windows 603 in accordance with one embodiment.PPS detectors 602 are not hermetically sealed, but instead open to thegas atmosphere and at the same pressure as the controlled chamberpressure. PPS 602 is in the form of a vertical stack. Gas input 605 andexit valves 606 control the gas flow, which can be maintained close tothe ambient pressure or the valves can be shut with no flow to maintaina stable static gas condition and then occasionally opened as needed tochange the gas pressure or exchange the gas volume. A slightly negativechamber pressure is shown in FIG. 6, but it could also be slightlypositive.

In addition to ultra-thin foils and metalized polymers serving as theouter-most walls of the external PPS enclosure, such as enclosure 401 ofFIG. 4, both metal foils or metalized polymers films could also serve asthe cover plate substrate for the PPS device itself within the describedenclosures. For this latter application, these materials would work withboth columnar-discharge (“CD”) and surface-discharge (“SD”) configuredPPS structures or embodiments.

Ultra-thin, UV-sensitive, PPS photon detectors can be fabricated withthe addition of a suitable internal photocathode or photoanode layer onultra-thin substrates having high UV (ultraviolet) or VUV (vacuumultraviolet) transmission. For example, the ultra-thin “flexible”glasses disclosed above are typically of an aluminoborosilicate typecomposition, and in thicknesses on the order of 100 microns have good UVtransmission down to about 260 nm. Alternatively, good VUV transmissiondown to about 155 nm can be achieved using high quality, ultra-thinfused silica substrates, or extended further for VUV transmission downto about 140 nm using the same thickness sapphire crystal substrates.Both fused silica and sapphire are readily available in thicknesses of127 microns in 4″ diameter substrates, and in thicknesses down about 25microns in smaller diameter substrates. Because of their hightemperature materials construction, such PPS detectors having highUV-VUV transmission can operate in harsh, high temperature environmentsin excess of 200° C., and perhaps in excess of 300° C. An example of onesuch commercial application for this type of detector would be a gasturbine flame-out sensor to monitor the UV signal produced by the hightemperature flame of a gas turbine engine used for electricalgeneration. The purpose would be for the sensor to “immediately” send anelectrical signal to shut off the gas supply under conditions of a gasflame-out in which the flame and accompanying UV signal aresimultaneously extinguished, and an explosion could result if the gassupply were not cut off immediately.

The ultra-thin PPS detectors as disclosed in accordance with embodimentscan lead to significantly improved performance for many applicationsover the current generation of solid state and gaseous detectors,including: resistive plate chambers (“RPC”), cathode-strip chambers, gaselectron multipliers (“GEM”), microstrip gas chambers/counters,Micromegas, etc. For example, important scientific and medicalapplications could be addressed by ultra-thin PPS detectors including:low energy, heavy particle, active pixel beam monitors; low attenuationand low scattering beam monitors for both photon and hadron particlebeam therapy; and particle (e.g., proton) CT imaging to complement oreven replace conventional X-ray CT imaging. For particle trackingincluding CT imaging, the use of ultra-thin devices allows a verticalstack of two or more such devices, such as shown in FIG. 6, to track anionizing particle that transits though the thin particle stack.

FIG. 7 illustrates the use of embodiments of the invention for medicalCT imaging. As shown in FIG. 7, a patient 700 is positioned between asandwich of ultra-thin vertically stacked PPS's, which could be a PPSdetector system similar to that shown in FIG. 6, with at least two PPSdetectors 702 in front of the patient and at least two such detectors704 behind the patient, thus being able to track a particle as it entersand leaves the patient. Such a system could be used for particle CTimaging, including proton CT imaging.

The refillable PPS test panel of FIG. 3 during testing has held gasmixtures that have operated for more than a year after the shut-offvalve was closed without any noticeable change in radiation detectionperformance. Similarly, embodiments employ periodic gas filling and“sealing” by closing the shut-off valve or valves on an ultra-thin PPSgas enclosure chamber such as shown in FIGS. 4, 5 and 6, without havingto maintain a continuous gas flow system. Although not “hermetically”sealed, such a valve-sealed chamber can operate for many months if notlonger before needing to be refilled. In another embodiment, a small gasreservoir of the discharge gas can be maintained either within orattached outside the PPS enclosure chamber with an associated gaspressure regulation system that could maintain a constant internal gaspressure regardless of ambient pressure variations. Alternatively theinternal gas pressure could be dynamically maintained to approximatelymatch the ambient pressure to minimize distortion of the ultra-thinchamber windows. In this later embodiment, the chamber is able to beself-calibrated with respect to operating voltage efficiency as afunction of internal device pressure. Embodiments rely on the majorityhost component to be one of the inert gases: He, Ne, Ar, Kr, or Xe, or avery stable host gas such as CF₄ or N₂, while one or more minoritycomponents could be either another inert gas or a suitable Penning orquench gas, for example: CO₂, CF₄, C₂F₆, etc., including Geiger tubetype quench gas compositions.

As disclosed, embodiments of the PPS utilize elements of construction ofthe PDP. PDPs consist of arrays of electrodes deposited on glasssubstrates, separated by a gas discharge gap. The electrode plus gapconfiguration constitute a cell or pixel. Commercial PDPs are comprisedof millions of cells per square meter, each of which can initiate andsustain a localized plasma discharge almost indefinitely at a typical“on” rate of 20 to 50 kHz. PDPs have been produced as both DC-type andAC-type units. Discharge termination in a DC-type PDP is aided by aquench resistor, while AC-type units exploit the gap field reversalproduced within dielectric layers deposited over the dischargeelectrodes. The essential structural components of a PPS include theelectrode materials, a suitable substrate (e.g., glass or ceramic),dielectric grids or windows or cavities, spacer plates or frames,strips, rods, balls, or a spacer cavity/hole plate structure typicallyof glass or ceramic or glass-ceramic, and the gas mixture that fills thegap between the two substrates and contained to form the panel or panelenclosure/chamber.

Embodiments of the PPS cell are biased to discharge when free-electronsare injected into or created in the gas. Such electrons then undergorapid electron avalanche multiplication, producing streamers that leadto a plasma discharge confined to the local pixel cell space. For auseful PPS device this process is self-limiting and self-contained byvarious means, one of the most important being an effective quench gasin combination with a localized impedance at each cell or on each line.The total charge available to produce a signal is limited by theeffective cell capacitance. The stored charge limits the maximum gain.The gain therefore depends on details of cell geometry but likelyexceeds 10⁶, because for panels tested to date the unamplified dischargesignal has been in the tens of volts, so high in fact that significantattenuation has been needed for the readout electronics. A typical 42″diagonal, high resolution PDP-TV (i.e., 1080p) has a cell pitch of about160 microns and a gas gap of approximately the same amount, although PDPcell pitches of 109 microns have been made for smaller size (i.e.,21-inch diagonal) higher resolution military display applications.Because of the small electrode gaps, large electric fields typicallyarise with something on the order of about a thousand volts of bias. Thecell is operated above the proportional mode and could be thought of asa micro-Geiger counter. The signal pulse is essentially independent ofthe number of initiating free electrons, rendering the PPS asintrinsically digital.

Embodiments can have applications to hadron particle therapy, protonCT-imaging and robotic surgery. Cancer is the second-largest cause ofdeath in the U.S., and approximately two-thirds of all cancer patientsreceive radiation therapy at some point during their illness. Themajority of radiation treatments are still performed with linearaccelerators (“linacs”) that generate energetic electron beams andX-rays. However, in recent decades particle beam therapy using protonand carbon ion beams has rapidly evolved into a new frontier in cancertherapy.

Although existing technology, such as intensity modulated radiationtherapy (IMRT) with photon (X-ray) beams, allows delivery of awell-defined, conformal dose to the target (tumor) volume, this can onlybe accomplished by directing the beams from multiple angles.Consequently, large volumes of normal tissue are exposed to low doses ofradiation from the multiple X-ray beams entering and leaving the body,increasing the risk for long-term side effects including secondarycancers.

Particle beam radiation therapy offers advantageous physical-dosedistributions (protons and heavier-ion beams) and potential for higherbiological effect in the target (heavier-ion beams) as compared tophoton radiotherapy. A considerable body of experimental and clinical,treatment-based evidence indicates that in certain settings particlebeams might be more effective in treating cancer as compared to the mostsophisticated photon-based therapies, while significantly reducing thevolume of normal tissue irradiated. One important requirement is theability to provide detectors that afford single-particle registration athigh data rates with a high degree of uniformity and minimalinterference with the particle beam. This would allow performing protonor ion CT prior to treatment and 2-D proton/ion radiography duringtreatment for integrated range verification, along with beam monitoringthat has minimal interference with the primary beam. The ability totrack individual particles at high rates with thin or ultra-thintracking detectors is therefore critical for particle or proton CTimaging. More specifically, the registration of individual particlesallows it to decide which proton histories to use for imagereconstruction and to predict their most likely path through thepatient. Based on these principles, images of high quality can bereconstructed with much lower dose compared to X-ray CT technology andaffording a much better prediction of particle beam range in tissues.One major disadvantage of the currently available tracking detectors forthis application is their limitations in size. Current silicon wafersare not available larger than 6 inches (15 cm), and therefore, largearea detectors have to be built from many smaller detector tiles. Thisis seen as a major limitation of the current proton imaging technologyboth for CT and radiography. Similar limitations exist with suchlarge-area sensors for intensity, position, and profile monitoring ofparticle beams during therapeutic delivery. Here it is very important tohave detectors with high physical uniformity and minimalwater-equivalent thickness to avoid spoiling the therapeutic beamproperties. Embodiments of the ultra-thin-PPS as disclosed constitute afamily of detectors that could be ideal for these applications.

The disclosed PPS embodiments have the potential to allow particle beamradiation therapy to realize its fullest potential and to be used infuture clinical particle beam radiation therapy facilities. In additionto particle beam therapy and proton CT imaging, the describedembodiments have broad implications for other fields that would greatlybenefit by ultra-low-mass detectors, such as the detection of high mass,low energy particles for basic nuclear physics research, as well as forhigh energy physics research and neutron detectors with low gammasensitivity.

For ion beam medical imaging applications, PPS detectors on the order of40×40 cm (i.e., active area) can be designed to satisfy the need formost large-field radiography and proton CT detectors. For suchapplications, a variety of embodiments would be based on an ultra-thinenclosure or chamber, which itself would contain one or more ultra-thin(i.e., ultra-low-mass) PPS devices, as shown in FIGS. 4-6. The thinnestglass used for commercial PDP devices is 1.6 mm, as all PDPs mustmaintain a hermetic seal and be thick enough to withstand ambientpressure differences without suffering glass bending/distortionresulting in non-uniformity of the gas discharge gap. However, most ofthe embodiments disclosed herein employ thin and ultra-thin substratesbased on ceramics, flexible glass, foils and coated polymer films. Forexample both fused silica and sapphire (R-plane Al₂O₃) are readilyavailable in 4″ diameter disks in a 0.127 mm thickness and available inthicknesses down to about 0.025 mm in smaller diameter sizes. Flexibleglass is available in much larger substrate sizes (e.g., large diameterrolls of several feet in width) down to the same thickness of 0.025 mm,and metal foils and coated polymer films are readily available inthicknesses of less than 0.005 mm. Embodiments can employ suchultra-thin substrates because they can be supported by an internal, orexternal, or combined (i.e., sandwich) internal/external grid-supportstructure, or they can be packaged within an ultra-thin window chamberenclosure that employs similar ultra-thin metallic foil or coatedpolymer films for the top, or top and bottom, wall structures within aninnovative static or dynamic gas flow system that maintains the chamberand panel at the same pressure regardless of the foil distortion due tothe external pressure difference, as shown in FIGS. 4-6. Because thereshould be “zero” pressure differential inside versus outside the panelfront and back substrates when packaged within the described ultra-thinwindow chamber enclosure, such devices should have extremely uniform gasdischarge gaps.

The disclosed embodiments have low mass and are inherently uniform withhigh temporal and spatial resolution. For example, temporal resolutionsof a few nanoseconds have been demonstrated, as has a spatial resolutionof 0.7 mm in PPS devices with 1.0 mm pixel granularity (i.e. cellpitch). Embodiments having sub-millimeter image resolution combined withultra-thin device structures and high particle detection efficienciescan make these detectors ideal for a number of particle tracking andbeam monitoring applications including CT diagnostic imaging andparticle beam therapy. The implications are quite profound as particleCT imaging could potentially eliminate the use of many if not most X-raytechnology modalities. For particle beam therapy, all of the imaging andtreatment could be performed entirely with charged particles, forexample protons within an energy range of approximately 50 to 500 MeV.The conversion of Hounsfield values produced by X-ray CT, to calculatingthe particle stopping power, is a well-recognized problem of proton andion beam radiation therapy that frequently prevents the stopping ofprotons or ion beams in front of a critical structure, thereby losing amajor promise of the Bragg-peak property of particle beams in cancertherapy.

It has long been known that particle imaging is an order-of-magnitudemore dose-efficient than X-ray based imaging, making routine use forimage guidance a realistic possibility. Effective large area, lowattenuation detectors such as ultra-thin PPS devices in combination withlow-intensity charged particle technology can allow diagnostic particleCT imaging with its faithful reproduction of electron densitydistributions to become available. This will allow very low doses (i.e.,sub-mSv for CT and μSv for radiography) for example for screeningstudies (lung cancer, breast cancer, kidney stones, etc.) as well as forreal-time, robotic, non-interventional surgery.

Two main advantages for proton-CT imaging are order-of-magnitudereduction in radiation exposure (i.e., ˜90%), and order-of-magnitudeimprovement in temporal resolution. For example, proton-CT (“pCT”)imaging should provide images similar to X-ray CT images, but with abouta 90% reduction in radiation exposure to the patient. This issignificant because CT-imaging causes cancer in some patients. Forexample, a conventional abdominal CT-scan (with contrast) is equivalentto about 200 chest X-rays and will cause something on the order ofbetween 1-in-500 to 1-in-1000 new cancers per patient scan.

In comparing X-ray CT to pCT imaging, approximately similar spatialresolution and area/volume coverage is expected, but with bettercontrast/density discrimination, much faster time resolution, andorder-of-magnitude lower radiation dosage at potentially comparablecost. In terms of time resolution, the fastest X-ray CT commercialscanner (e.g., the 640 slice Toshiba Aquilion ONE VISION) can do arotation in about 0.3 seconds, which although seemingly fast is not fastenough to prevent some potential image blurring from heart movementduring contraction, whereas pCT imaging has the potential to capture animage perhaps a 1000 times faster (˜0.0003 seconds).

Unlike most solid-state radiation detectors, PPS detectors in accordancewith embodiments are inherently rad-hard, and can be designed to detectboth high-energy X-ray beams and particle beams with minimal beaminterference (e.g., scattering). PPS detectors have the potential tomeasure the incident beam intensity distribution and beam shape inreal-time as the patient is undergoing cancer treatment, while beingalmost transparent to the beam transmission through the PPS detector.The beneficial result is that use of PPS detectors should result in verylittle deviation of the beam on its path to the target tumor site (i.e.beam scattering to adjacent healthy tissue).

The benefits of very low radiation dosage coupled with very hightemporal resolution enables a new type of interventional radiology(neurovascular, hepatic, etc.) or particle-CT fluoroscopy that couldlead to real-time, non-interventional robotic surgery which could berevolutionary for a number of surgical procedures. For example,real-time pCT could enable image-guided robotic surgery via a threadedcatheter which could repair the heart or other organs non-invasively.This is not possible using X-ray CT imaging because the patient would beexposed to too much radiation during such a procedure and the temporalresolution of X-ray CT is not fast enough. For heart surgery thiscapability could enable time-resolved (i.e., 4-D) real-timecardiovascular CT surgery, but could also be used for other types ofcardiology (e.g., coronary, structural, electrophysiology, etc.) as wellas real-time, image-guided robotic endoscopy procedures, etc.

In one embodiment, the ultra-thin PPS uses a discharge-gap closelycoupled to an orthogonal ion-pair creation drift region 818. FIG. 8 is aperspective view of an ultra-thin surface-discharge PPS pixel array inaccordance with one embodiment. The front substrate 802 contains adrift-field electrode 822. The X and Y lines, 824, 826, on the backsubstrate 814 define cells with embedded resistors 830 and avalanchedischarge gap regions 820. The front substrate could be made off anultra-thin metalized polymer film, or an ultra-thin metalized ceramicmembrane such as Si₃N₄ or alumina, or an ultra-thin metalized flexibleglass or fused silica, or an ultra-thin metal foil such as an aluminumor titanium or cobalt-alloy foil.

In the embodiment of FIG. 8, the ultra-thin front substrate 802constitutes the cover plate and can be fabricated from either anultra-thin metal foil (e.g., aluminum or titanium foil), or anultra-thin metalized insulator substrate. In the latter case themetalized ultra-thin insulator substrate could be made of: (1) a polymerfilm (e.g. Mylar, PEN, PEEK, etc.), or (2) an ultra-thin transparentinorganic substrate such as glass, fused silica, sapphire, etc., or (3)an ultra-thin ceramic such as Si₃N₄ or alumina, etc. The electrodeconfiguration on the back substrate 814 is defined by a local electrodearrangement forming a capacitive discharge gap 820 coupled to anembedded resistor 830 in the high voltage feed lines. The resistancereduces the electric field during discharge and terminates the pulse.The effectiveness of this resistance in terminating the discharge hasbeen experimentally demonstrated. Some features of the design conceptare as follows: (1) Pixels can be defined in either an open structure asshown in FIG. 8, or an “open” cavity or grid-hole structure as shown inFIGS. 9 and 10, which is optically, physically and electrically isolatedwith the pixels established at the crossing points of electrodes asshown in FIG. 10; (2) Pixels can be close packed assuring high coverage(i.e., high fill-factor); (3) The gas envelope can be formed byultra-thin, low mass, substrate materials such as glass, ceramic, metalfoil, or even inorganic coated plastics; (4) Cavity geometry can beoptimized for efficiency and discharge voltage; (5) Fabrication utilizeswell established processes many of which are used in the production offlat panel displays; (6) A hard, sputter resistant surface that is veryresistant to ion damage would likely be chosen for the cathode, such asa high temperature, corrosion resistant, refractory type metal coating(e.g. Ni, Cr, Ti, Zr, Pt, Mo, W, Ir, Ti—W, etc.), but could alsoincorporate a conductive or even semiconductive non-metal with similarphysical properties such as B₄C, TiN, etc.

FIG. 9 illustrates a fabricated alumina grid-support plate 900 inaccordance with one embodiment. Plate 900 is a 25.4×25.4×0.38 mmfabricated alumina grid-support plate with 120 laser cut, 1 mm diameterholes (e.g., 901, 902) located within a 13×13 mm active area, yielding afill factor ˜60%. The grid-support plate hole pattern for vacuumenvironments will sandwich/support the ultra-thin, ultra-low-mass windowfrom both sides, whereas for operation under ambient pressure thegrid-support hole plate supports the window from the inside since thegas pressure will be negative with respect to the outside ambientpressure. Window thicknesses can be as thin as 1 μm for Si3N4 or 1.5 μmfor a double-sided metalized film of polyethylene naphthalate (PEN).

FIG. 10 is a two-dimensional perspective drawing of a grid-support PPSstructure 1000 in accordance with embodiments of the invention.Structure 1000 has a 56×56 mm back plate and 38×38 mm front plate. Theactive cell area is 13×13 mm, and shown with 120 grid support plate cellholes (each 1 mm diameter), having the same pattern as in FIG. 9. Pixelsin FIGS. 9 and 10 (e.g., pixels 1010 of FIG. 10) are close packedassuring high coverage (i.e., high fill-factor).

PPS device manufacturing employs a number of similar processes used forthe production of LCD and PDP flat panel displays. The primarystructural components include inorganic substrates (e.g., glass),dielectric printed window frame type structures and/or barriers,electrode depositions, gas filling and envelope sealing. Large panel21-inch diagonal AC-PDP's with a cell pitch of 109 μm were fabricated inthe early-1990s for high resolution applications. The structure of theseAC-PDP's was in many ways more complex than the described PPS devices,as the former display panels involved difficult-to-control, thick-filmscreen printing of sequential RGB phosphor patterns requiring criticalpixel alignment on “thermally unstable”, high expansion, soda-limesilicate window glass substrates, as opposed to today's highlyengineered, much more stable, display glass substrates. The PPS devicesin accordance with embodiments do not require such difficult-to-controlpatterning processes on “thermally unstable” substrates. Additionally,these devices are based on “bare” electrodes as anode and cathode. Thisallows diminutive line widths and pitches. In fact the smaller linefeatures on a typical smart phone display has ≤1.5 μm electrodes with≤0.3 μm alignment accuracy. The line width uniformity can be achievedfor example by conventional thin-film photolithography patterningincluding ion-milling etch processes, similar processes can be used for100 μm pitch PPS electrodes. The cavities themselves can be fabricatedby a variety of methods well known to those in the field, includingchemical etching, thick-film patterning and laser drilling.

FIG. 9 illustrates an example of laser drilling. FIG. 11 illustrates anexample of a grid-support cavity array 1100 formed with thick-filmpatterning in accordance with one embodiment. Cavity array 1100 isproduced by a UV-assisted roll forming process used in PDP fabrication.Rib walls as narrow as 20 μm have been fabricated using this process.

For many applications, cavity structures are not needed at all, and infact could be counter-productive by reducing the device efficiency, asthe thickness of the cavity walls creates dead space that reduces thedevice fill-factor. In such “open” configurations, narrow dielectricstrips or thin window frame type structures could be deposited orprinted over at least one of the discharge electrodes to prevent or atleast minimize discharge spreading. Alternatively, dielectric wallscould be fabricated between discharge electrodes and could even becombined with the above dielectric strips or windows frame patterns toform a more defined discharge cell/pixel structure. Depending upon theapplication, these dielectric strip, window, or wall structures need notbridge the gas gap, and therefore need not serve the dual function assubstrate spacers. If dielectric walls are used, they could even havevarying heights with some sections being open and other sections actingas pedestals or substrate gas gap spacers. Many variations of suchdielectric structures can be made and in fact are commonly used today incommercial DC-PDPs for the same purpose of confining or localizing thegas discharge. Such devices might be able achieve geometric fill factorsthat could approach 100%, and when combined with ultra-thin substrates,could potentially result in ultra-thin, ultra-low-mass, particledetectors having detection efficiencies on the order of 95% or possiblyhigher. In particular, by constructing “open cell” structures with largegas gaps and operating at sufficiently high fields, device efficienciesapproaching 100% should be attainable.

The materials requirements for embodiments of the invention aredetermined primarily by radiation hardness and aging resistance.Materials intended for use are intrinsically non-degrading with exposureto UV/VUV photons, ionizing radiation, and ion bombardment at thecathode surface. The described PPS devices, like PDPs, incorporateinert, non-reactive and sputter resistant components. Examples of thethree most critical materials follow.

Substrate: The substrate is ideally comprised of ultra-thin, very lowgas permeability, flat panel display glass, although ceramic substratessuch as alumina or engineering glass-ceramic or ceramic type materialsare also suitable. An example is Corning Eagle-XG Slim™ Display Glass,commercially available in meter sizes and larger and in thicknesses of0.3 mm and thinner. Other glasses by Corning such as flexible “Willow”glass are commercially available as rolls in thicknesses of 0.1 mm and0.2 mm roll, and Schott is making ultra-thin glass in thicknesses of0.025 mm. Other substrate materials include 0.001 mm thick Si₃N₄, 0.125mm fused silica and alumina, metal foils and polymer films on the orderof 0.01 mm. For gas permeable substrates such as polymer films, etc.,the ultra-thin PPS would itself have to be enclosed in a polymer film ormetal foil based single or multi-walled window chamber/enclosure filledwith the same gas as shown in FIGS. 4-6, at the same gas pressure as thePPS.

Metal Electrodes: Metal electrodes are subject to continuous ionbombardment and should be composed of sputter resistant, refractory typematerials. One indicator of the strength of the metallic bond to resistion sputtering is the melting and boiling point temperature. Promisingchoices for an electrode material includes: Ni, Cr, Ti, Pt, Ru, Zr, Ir,Mo, W, Ti—W, etc.

Gas Fill: PDPs are hermetically-sealed with demonstrated remarkably longlifetimes. Units sold in the 1970's and operating continuously (24/7)are still functioning today, more than 35 years later. This leads to adesign objective of PPS detectors: the gas remains sealed inside theenvelope without the need for an external gas system, although this maynot be desirable for ultra-thin PPS devices in which the panel envelopeis effectively extended outward by being “packaged” within an externalenclosure/chamber with at least one ultra-thin wall/window. The PPSdischarge gas should ideally be a stable or inert host gas either byitself or with one or more added components such as a quench gas. Thechoice of gas mixture may also be dictated in part by the need tominimize internal sources of free-electrons that might collect on thewall surfaces. In some embodiments a method demonstrated to inhibitunwanted free-electrons can be employed to minimize the number ofgas-phase metastables, reducing lifetimes of gaseous excited statespecies. This involves the addition of a suitable Penning gas dopantwith good electron affinity, a large electron capture cross-section, andVUV absorption. Dopants could include: Xe, CO₂, N₂, CF₄, SF₆, etc.Hydrocarbons such as C₂H₆, C₃H₈, i-C₄H₁₀, etc. are generally unsuitableas their decomposition would be problematic for those embodiments thathave a hermetically sealed structure. Finally, for PPS configurations inwhich it is desirable to minimize the gas gap, which could includecertain ultra-low-mass PPS structures with minimal internal dielectricstructures, gases having large radiation interaction cross-sectionswould be favored such as Xe, CF₄ and SF₆.

PPS devices that incorporate gas flow-through systems should bepractical for a number of ultra-thin detector applications includingactive-pixel ion beam monitors, low energy charged-particle detectors,low scattering hadron particle medical imaging systems such as proton CTimagers, detectors and dosimeters for X-ray and proton therapy, etc.Because of potential gas permeation issues in the thinnestconfigurations of some embodiments of ultra-thin PPS detectors, forexample devices made with substrates in the range of approximately 1-20microns, such devices might require continuous or periodic gas exchangein a flow-through detector design. In some such embodiments theultra-thin PPS could be configured with an “optional” second exhaustvalve. FIGS. 12A-C illustrate an ultra-thin PPS with a second exhaustvalve in accordance with one embodiment and a surface-dischargeelectrode structure in accordance with another embodiment. FIG. 12C isfor a surface-discharge electrode configuration in which pixels 1250 areformed from segmented cathodes 1260 coming from a cathode bus-bar 1270on the bottom surface up to top surface of the back substrate through aconductive via (not shown), and paired in close proximity to anode lines1280 on the top surface of the back substrate.

Many electrode configurations are possible, some based oncolumnar-discharge structures, others based on surface-discharge and/ormicrocavity-discharge electrode structures. In the example shown in FIG.12B, the electrode structure 1204 would be based on line anodes andcathode segments with “optional” quench resistors in each cell. Suchstructures would share a number of similarities with variousmicrocavity-PPS or cavity grid-support PPS back plate resistor/bus-barconfigurations. The cavity grid-support structure could be fabricated,for example, as an intermediate rectangular cavity/spacer plate with thethin-film anode traces running under the cavity spacer plate and throughthe perimeter seal. The intermediate cavity plate could have a sealgroove on both the top and bottom surfaces to accommodate the sealmaterial without spacers and thereby sealing the back and front platesflush to the cavity grid-support structure. FIG. 12B shows theorthogonal “vertical” cathode bus-bar layout 1204 on the bottom surfaceof the bottom substrate, with the “horizontal” anode line structureshown on the top surface of the bottom plate. A similar cavity seriesresistor layout used for microcavity-PPS devices can be used here forthe surface-discharge (“SD”) structure shown in FIG. 12C. The describedstructures would employ conductive cover plates that could be either anultra-thin metal foil or metalized polymer film or metalized ultra-thinceramic, all of which would be in a thickness range of ˜1-20 microns.

As disclosed above, ultra-thin PPS detectors can be fabricated where theelectrodes 1204 shown in FIG. 12B are configured for acolumnar-discharge arrangement. In such an arrangement one set ofelectrodes would be on the top substrate and one set on the bottomsubstrate. For example, the “vertical” electrodes could now be thecathodes and located on the top surface of the bottom substrate with aquenching resistor at the end of each cathode line positioned outside ofthe seal located to the right in FIG. 12B. Conversely, the “horizontal”electrodes would now be the anodes located on the bottom surface of thetop substrate. Unlike the drawing for the surface-discharge descriptionin FIG. 12, the top substrate would now have to extend beyond the bottomsubstrate to gain access for electrical connection to the anode. Theactual columnar-discharge electrode layout is best illustrated in FIG.1.

The use of ultra-thin PPS devices in accordance to embodiments forneutron detection is of significant benefit for additional reasons thanpreviously disclosed. For neutrons, the thickness of the substratematerials of the common display panel glasses, ceramics (e.g., alumina),metals foils (e.g., Al and Ti) and polymers has very little effect, ifany, on neutron attenuation or scattering. However the substratethickness has a large effect on gamma attenuation/interaction includingscattering, and therefore on the neutron-to-gamma discrimination ratiowhich is critical for most neutron detector applications.

In one embodiment, the PPS device embodiments disclosed herein can bemade into neutron detectors by either of two means. The first methodinvolves simply filling the PPS detector with a gas mixture thatcontains a significant amount of ³He or ¹⁰BF₃ as the active neutrondetecting media. Thus even for basic DC-PDP commercial panels that havebeen modified to function as charged particle detectors, such as thatshown in FIG. 3, by filling these devices with a ³He or ¹⁰BF₃ gasmixture they can become a good neutron detector. The second method is todeposit a thin-film layer of an efficient neutron absorber, such as ¹⁰Bor ^(155,157)Gd (or even an appropriate ⁶Li compound), on selectedinside surfaces of the PPS and then operate the device as a conventionalparticle detector. In this manner, the emitted charged particles from¹⁰B (e.g., alphas and Li ions), or ¹⁵⁵Gd and ¹⁵⁷Gd (e.g., conversionelectrons) will initiate the gas discharge in the normal fashion and bedetected with a standard discharge gas mixture (e.g,. CO₂ in Ar, etc.).The above neutron absorbing ¹⁰B or ^(155,157)Gd could even be coated ontop of either the anodes, or cathodes, or both, or even adjacent to theanodes or cathodes, or along the sidewalls of the grid-support or cavitystructure. Because only thin layers of such neutron absorbers can becoated per PPS device, to realize high overall neutron detectionefficiency such PPS neutron detectors would need to be verticallystacked in a multilayer configuration or have a very high aspect ratioof cavity hole-height to hole-diameter.

FIG. 10, and FIGS. 12, 14 and 15 illustrate the design of two differentgrid-support PPS structures. FIG. 12 illustrates a surface-discharge PPSgrid-support structure, while FIGS. 14 and 15 illustrate acolumnar-discharge PPS grid-support structure that would incorporate aseries resistor on each cathode line as in FIG. 2, while FIG. 10illustrates a grid-support structure which could incorporate a quenchresistor either in each cell, or on each cathode line outside the sealarea. A grid-support cell hole test plate with the same 13×13 mm activearea as shown in FIG. 10 is shown in FIG. 9 (the holes were laserdrilled). It is noted that the active area cell fill-factor in FIGS. 9and 10 is ˜60%.

FIG. 13 illustrates the bottom substrate 1300 of a PPS structure thatincorporates a cathode cell quench resistor located on the back side ofthe bottom substrate, with one resistor for each cell, in accordancewith one embodiment. The electrode structure on the backside in FIG. 13would connect to a cathode on the topside of the back substrate by meansof a laser or mechanically drilled conductive via. A higher cellresolution version having a higher fill-factor could be achieved byreplacing each thick-film conductive via with a thick-film resistive viaconnecting directly to the backside cathode bus-bars, therebyeliminating the surface-mounted-technology (“SMT”) resistors shown inFIG. 13.

The backside of substrate 1300 is a 4 mm cell pitch, 56×56 mmgrid-support PPS back plate showing discrete SMT resistors 1304 solderedto their pads connected to each circular cell HV-cathode on the topsideby a conductive via going from the backside to front side. FIG. 13 alsoshows zig-zag cathode bus-bars 1302 that terminate at the edge cableconnector 1310. A vacuum stainless steel hex-fitting (to thegas-exhaust/gas-fill line) 1306 is shown in the top right corner.Alignment holes 1305 are shown next to connector 1310.

For some of the above disclosed embodiments, a series cell quenchresistor can bridge the high voltage bus on the backside to a “circular”cathode on the topside centered inside the circular grid-support platestructure shown in FIGS. 9 and 10, thus establishing independent readoutsites along one coordinate. The sense/anode lines on the inside surfaceof the ultra-thin window provide an orthogonal coordinate readout. Thisgrid-support structure can be made using standard PDP thick-film cavityfabrication technology directly onto the back substrate after thethin-film electrodes and vias have been fabricated, or laser drilling ona grid-support plate, or by mechanical milling or other means such as byphotolithography and chemical etching, etc.

FIG. 14 is a perspective view of a grid-support PPS structure 1400 inaccordance with one embodiment. Structure 1400 includes a top plate1401, an intermediate grid-support plate 1402, and a bottom plate 1403,along with gas fill tubing 1404 hermetically sealed to a panel gas-fillhole 1410 and connected to a gas fill/shut-off valve 1405. In theembodiment of FIG. 14, which has a columnar-discharge type PPS electrodestructure, the 20 vertical column electrodes on the bottom surface ofthe top substrate are not shown (although they are shown in FIG. 15),but the 20 orthogonal horizontal row electrodes on the top surface ofthe bottom substrate are shown. Proper alignment of the three plates1401-1403 is achieved using an external alignment fixture with alignmentpins that fit through the respective alignment holes 1444 and alignmentslots 1446. Of course other alignment schemes can also be employed suchas a variety of alignment fiducials and/or edge grooves or othermarkings, etc.

FIG. 15 is a top view of a more detailed composite overlay view of thethree plate PPS structure of FIG. 14 in accordance with one embodiment.FIG. 15 illustrates the orthogonal 20 column top substrate and 20 rowbottom substrate electrodes, forming 400 pixels, shown along with theirrespective electrode pad connector pattern located for the top plate onthe bottom edge, and for the bottom plate on the right edge. FIG. 15also illustrates a major horizontal gas flow channel artery 1520 thatconnects each of the 400 shown individual cavity holes that define eachpixel 1510, via a matrix of connecting gas flow notches between adjacentcavities (not shown), to the gas fill hole 1530 located near the topleft corner which is sealed to the gas-fill tubing and tube connectionhardware 1540 to the gas-fill/gas shut-off valve 1405 shown in FIG. 14.A double-sided hermetic seal 1550 is also shown in FIG. 15 between boththe top surface of the bottom plate and the bottom surface of the middlegrid-support plate, as well as between the top surface of the middlegrid-support plate and the bottom surface of the top plate. In addition,at the two bottom corners of the panel in FIGS. 14 and 15 can be seen apin alignment hole on the bottom left 1444 and a pin alignment slot onthe bottom right 1446. In the embodiment of FIG. 15, the seal channel1550 to the bottom edge makes a 45 degree angle so as to keep the sealaway from the external plate alignment holes in order that the holes notprovide a leak path into the sealed panel gas volume.

The closed cell, grid-support PPS device embodiments disclosed abovehave at least three particularly important advantages in contrast toknown devices: (1) they can structurally support an ultra-thin windowthat will transmit very low energy, ion beam nuclei with both highposition and high temporal resolution, (2) the closed cell structurephysically isolates each cell from its neighbors thereby eliminatingcell crosstalk due to migrating metastable excited species and VUVphotons from causing secondary discharges in neighboring cells, and (3)the grid-support structures act like a matrix of tubular spacerssurrounding each cell like a honeycomb structure and result in anextremely uniform gas/discharge gap which directly translates into auniform field with uniform discharge characteristics including bothtemporal response and pulse arrival time distributions. Since thegrid-support substrate surfaces can be easily machined or lapped flatand parallel to within about ±3 μm or better if polished, this wouldrepresent a gas discharge gap non-uniformity of just 0.3% for a 1 mmheight grid. As shown in FIG. 13, surface mount resistors 1304 canbridge each inside cell cathode to an external high voltage bus 1302.The grid-support plate 1402 in FIG. 14 can be laser or mechanicallymachined or photo-chemically etched from previously described glass orceramic or glass-ceramic substrates. Assembled devices can be eitherhermetically-sealed, as shown by the perimeter seal 105 in FIG. 1, orsealed with a gas-flow system as shown in FIGS. 4-6 and 12. The anodesin FIG. 10 or 12C or 14 are at ground potential, and connect toX-coordinate readout electrodes. Capacitive coupling to the HV lines, ordirect readout, can deliver the Y-coordinate signal. X-Y ambiguities canbe resolved with fast signal time stamps. For very large areawindows/devices, fabricated using either double-sided metalized films ona polymer substrates, or ultra-thin metal foils, the grid-supportstructure can be inexpensively fabricated via thick-film technology suchas shown in FIG. 11. These and related precision processes are commonlyused in the fabrication of PDP barrier rib structures.

Plasma discharges can initiate other discharges to regions beyond thecell. The precursor avalanche produces UV photons that can propagate toother regions where photoelectric ejection or even direct ionization ofthe gas can occur. Excited metastable species can also propagate andcause discharge spreading. Both of these mechanisms can be eliminated bya matrix of barrier or grid-support walls surrounding each cell toprevent propagation of VUV photons and metastables. Virtually all PDPtelevisions have internal barrier structures, with the fabricationprocesses for these structures well-established (e.g., as shown in FIG.11). Alternatively, a proper choice of gas quenching and/or VUVabsorbing agent and electrode materials can be effective. Penning gasmixtures are a tried and true method to depopulate metastables bycollisional energy transfer resulting in ionization of the lower energystate component. In addition, polyatomic molecules by their very nature(i.e., being multi-atom), can potentially be effective agents fornon-radiative energy transfer and internal conversion, and can also begood VUV absorbers. As disclosed above, even with open-structured PPSdevices (i.e. no internal barriers, as shown in FIGS. 2 and 3),discharge spreading to adjacent cells can be minimal with properselection of the discharge gas and quenching resistor. With regard tothe cell circuitry, the use of a properly sized series resistor canquickly drop the cell or line voltage, thereby collapsing the field andtruncating the discharge. A combination of the above factors has provento be effective in addressing and managing the problem of dischargespreading. This problem should be essentially eliminated by enclosingeach discharge site in an optically and electrically isolated cavity(e.g., the grid-support PPS structure).

Using modified-PDPs, six different gas systems have been studied,namely: Ar+CO₂, Ar+CF₄, He+CF₄, CF₄, SF₆, and Xe. Not unexpectedly,large variations in device response and performance are observed for thevarious gases and pressures. The device performance has been shown to bevery much gas dependent, with the breakdown voltages varying by morethan 1000 volts for different gas mixtures in the same panel. It isclear that different applications will require different gas mixtures.The gas pressures studied to date range from 200 to 730 Torr. FIG. 16 isa graphical illustration of volts vs. time in nanoseconds illustrating acharacteristic PPS signal induced by a beta source in accordance withembodiments of the invention. The signal of FIG. 16 is observed on acommercial PDP filled with 600 Torr Xe (in 2003). Note the largeamplitude volts level signal and nanosecond level rise time (measured in2010). In general, as shown in FIG. 16, the signals observed from all ofthe gases tested have large amplitudes of at least several volts, sothere generally is no need for amplification electronics and in mostcases the signals have had to be substantially attenuated. For each gasthe shape of the induced signals are substantially uniform, andinherently digital. The rise time is typically 1-2 ns, the FWHB varies,but for some gases has been measured as 2 ns.

The discharge spreading to neighboring cells is very much gas dependent,but even in an “open” cell structure the discharge spreading can beminimized even though these devices operate in the Geiger mode,producing large amplitude, high gain discharges. Significantly, this hasbeen done without an internal barrier structure around each cell, andthe devices do not require the addition of a hydrocarbon quenching gascomponent that would certainly degrade in a plasma dischargeenvironment.

For grid-support PPS devices in accordance with embodiments, fasterresponse times are anticipated (i.e., in the sub-nanosecond range) withsmaller cell dimensions, better cell physical and electrical isolation,and lower panel capacitance. The grid-support wall structure, as shownfor example in FIGS. 9, 10, 14 and 15, in physically isolating each celleffectively prevents both gaseous metastable species (i.e., long-livedexcited state atoms) and VUV-UV photons created in the discharge fromspreading to adjacent cells, thereby further reducing cell crosstalk andenhancing the temporal resolution. As disclosed above, huge performancevariations are associated with different gas mixture compositions andpressures. The fast response times observed to date are assumed to belargely due to the very high gain of the PPS Geiger-mode electronavalanche, which is likely generated via a “micro-streamer” gasdischarge type mechanism. The grid-support PPS structure, with very thinwalls between adjacent cavities, offers the potential for greatlyincreased efficiencies and much better temporal resolution (in additionto the obvious enhanced position resolution). This is because thegrid-support structure can offer a much higher proportion of active gasvolume between the cathode and the sense anode. Achieving spatialfill-factors of 80% to 90% can be achieved using standard fabricationprocesses known to those skilled in the art. For example, since thecavity/holes can be fabricated by a variety of processes, includinglaser, photolithographic, and mechanical/ultrasonic patterning, as wellas numerous variations of thick-film technologies, as shown for examplein FIG. 11, grid-support/cavity structures of almost any shape ordimension can be made down to ˜50 μm or less (e.g., hexagons, invertedtriangles, diamond shapes, modified ellipsoids, etc.).

Further, advantages are achieved based on the uniformity of the gas gap.For example, in constructing a grid-support cell structure by thesubtractive method of removing the hole material, a thin wall ofremaining dielectric material of uniform height is left around eachgrid-support cell. More specifically, the grid-support structure shouldresult in almost perfect uniformity with respect to the discharge gap(e.g., about ±0.3%, as compared to at least a 20% variation in gas gapacross some of the modified-PDPs as in FIG. 3. This huge improvement ingas gap uniformity is because the grid-support PPS gas gap will now bedefined by the height uniformity of the grid-support walls, which inturn depends on the substrate thickness uniformity which can be lappedflat and parallel to within standard tolerances of 2-3 μm, or better ifneeded—e.g. critical optical parts can be fabricated to hold a thicknessvariation over a 6″ diagonal of within 0.003 μm. A 3 μm height variationin a 1 mm height cavity represents a gas gap variation of 0.3%. Yet evenwith the non-uniformity observed in the modified-PDPs, such as shown inFIG. 3, a position resolution of 0.7 mm was able to be demonstrated in apanel with a 1.0 mm pixel pitch.

Further, beam diagnostic and tracking detectors are especially criticalin heavy ion beam experiments. At the very least, experimenters needhigh efficiency, fast detectors to count beam particles incident on thetarget. For some experiments the beam position just before the targetmust be measured, as well as the time-of-flight of the beam particle.The requirements on beam counting and timing detectors include highefficiency and very fast rise time, preferably less than 1 ns, and verylow dead time to allow counting beams available in the next generationof radioactive ion beam (“RIB”) facilities. For tracking, positionresolution of better than 1 mm is required, along with small relativeenergy loss in the detector so as to minimize the perturbation of thebeam. In this regard, current PPS devices have already achieved positionresolutions better than 1 mm.

Intensity profiles and emittance analyses are among the most criticaltools for optimizing beam transport through accelerators. The PPS ishighly position and intensity sensitive (i.e., intensity via number ofcells firing repeatedly). Most beam experiments require minimum detectorinteraction, which means sufficiently thin detectors with low mass topermit maximum transmission of the beam with minimal energy loss. Deviceoptimization therefore requires a careful balance between substratematerial, gas composition, pressure, and gas path length. The higher thedevice efficiency, the less gas needed for suitable ion-pair generationand therefore a lower internal gas pressure thus allowing a thinnersubstrate with high beam transmittance in the accelerator vacuumenvironment.

The PPS in accordance with the disclosed embodiments further has thepotential to serve as a rugged beam position and intensity monitor incertain applications where there are few useful solutions at present. Inparticular, operators of the current and next generation RIB facilitiesneed monitors able to count from one particle per second up to a fewtens of millions while delivering accurate beam position information.The simple design of the PPS, using amorphous materials and an inert gasinteraction media is likely to withstand radiation damage more easilythan semiconductor detectors. It should also have a longer life thansolid-state electron multipliers (e.g., microchannel plates).

The particular requirements for different facilities will requirevariations in the thickness and size of the position sensitivedetectors. At lower energy facilities, E<10 MeV/A, the beams aregenerally small and the diagnostics are 2 to 5 cm wide. For continuousmonitoring at these facilities, the detectors must be extremely thin soas to minimally distort the divergence or energy of the beam. At higherenergy facilities, E>20 MeV/A, the beams are often dispersed over alarge area, requiring devices from 10 to 50 cm wide, as at the NationalSuperconducting Cyclotron Laboratory (“NSCL”). Thickness is somewhatless of an issue for these facilities.

Embodiments include electronics similar to the detectors disclosed inU.S. Pat. No. 7,332,726, and U.S. Pat. Pub. No. 2010/0265078, whichcount each discharge pulse as an event, and then base an amount ofdetected radiation on the number of counted events. Electronics can alsouse time-stamp circuitry so that a single pulse is not counted asmultiple events. Embodiments provide higher signal rates, faster timing,and more precise positional information than most other ionizationsensing devices, but are expected to be similar with respect totriggering and readout possibilities. The speed of these devices reducesthe probability of fake or ambiguous association of hits from the twoorthogonal readouts.

FIG. 17 is a block diagram of pulse-counting type electrode circuitry1780 for detecting each gas discharge cell interaction and counting eachsuch interaction as an individual pixel discharge event in accordancewith one embodiment. Circuitry 1780 can be coupled to the PPS devicesdisclosed above. In FIG. 17, X- and Y-electrodes, 1781 and 1782, areshown as being in an orthogonal arrangement (i.e., rows and columns).However the X- and Y-electrodes are not restricted to orthogonalpatterns and can be configured in any such way consistent with beingable to count and record pixel discharge events. Further, the countingcircuitry used with the PPS, disclosed in U.S. patent application Ser.No. 11/155,660, the disclosure of which is herein incorporated byreference, may also be used with embodiments of the present invention.The circuitry disclosed above can utilize any one of a number ofcurrent-limiting impedance component arrangements with regard to thetwo, gas-discharge electrodes (e.g., variously described as X and Y, orcathode and anode, or row and column), which are in-turn coupled to theevent counting detection electronics shown in, for example, FIG. 17.

The various embodiments of the detectors of the present invention asdisclosed herein, and as illustrated by the circuitry shown in FIG. 17,are fundamentally digital in nature and as such the detectionelectronics does not record the magnitude of a given cell discharge asdoes most “conventional” detectors operating in the linear region asproportional detectors, but instead operates in the non-linear regionand can employ Geiger-Mueller type counting techniques/circuitry, thusassigning essentially the same value to each event regardless of thecell discharge magnitude. Embodiments of the present invention can usecircuitry to acquire pixel discharge data by utilizing standard pointscanning, line scanning, or area scanning techniques.

Circuit 1780 further includes a discriminator 1784 to produce logicpulses which can then be fed to an array of field-programmable gatearray (“FPGA”) logic arrays 1786. FPGA arrays 1786 can perform thecalculation of the position for each hit, and emit a stream oftime-stamped (X,Y) coordinates. Discriminators 1784 must therefore beable to identify multiple hits on each electrode, and send this countinformation to FPGA 1786.

In one embodiment, to accurately record the number of hits on a wire ifthey get too large, the readout electronics are organized via a gridtype of architecture that monitors, records and integrates theindividual event counting results from a number of smaller sub arrays,thus requiring that more wires be brought out to reduce the number ofcoincident events along an extended length of electrode wire. Bringingout more wires require more, but simpler, discriminators.

In one embodiment, chains of connected (or isolated) cells withindividual cell or line quench resistors on the high voltage cathodeline establish the readout sites along one coordinate (e.g., the X-line)on one substrate. Parallel chains of connected sense lines or anodesprovide the second orthogonal coordinate (Y-line) readout. For thisembodiment each cathode line is connected to the high voltage DC powersupply with standard pulse discharge event detection circuitry connectedto one or both electrodes.

Embodiments operate as highly-pixelated digital radiation detectors byflashing “ON” each pixel (which is normally “OFF”) as a directconsequence of a gas discharge avalanche stimulated within the cell byincoming radiation, and so at their most basic level functionally behaveas digital radiation counters and not as proportional counters. Eachsuch gas discharge pulse is counted as having an approximately equalvalue (which has been experimentally verified numerous times) by adischarge event detector and is therefore counted by the circuit assimply an individual event. The amount of detected radiation is thusbased on how many individual gas discharge events are outputted from thepixels. The electronic readout circuitry is thus designed to detect ifand when a gas discharge pulse is outputted from the pixel (i.e., when apixel has turned “ON”). In order to maximize the temporal resolution,the readout circuitry preserves the cell discharge output pulse risetime.

In this environment, very fast synchronous digital signal processing isdirectly applicable and can be flexibly implemented in large and fastFPGAs. FPGAs also provide largely parallel processing which furtherextends the data rates that can be handled. The processing of thesesignals can be via synchronous programmable logic, but for moredemanding applications a conventional fiber-optic readout can be usedwith tracking and trigger algorithms.

The speed of disclosed embodiments reduce the probability of fake orambiguous association of hits from the two orthogonal readouts. Therelatively large signal size also distinguish embodiments from thosethat require high gain amplifiers prior to the hit processing. In thisenvironment, very fast synchronous digital signal processing is directlyapplicable and can be flexibly implemented in large and fast FPGAs.

Embodiments can exhibit single cell position resolution with nanosecond(“ns”) and possibly sub-nanosecond timing resolution for hits separatedby a dead time (i.e., recovery time) of the order of several ns. Forembodiments requiring fast timing, the PPS cell size should berelatively small, and can approach something on the order of a hundredmicrons. With the small cell size, fast sampling, and recovery time,embodiments can potentially deliver per square centimeter hit rates ofmore than 100 MHz. Association of hits on the two axes at this ratewould generate many ghost hits were it not for the fast timing. Forexample, if a 2 ns coincidence between coordinate axes is assumed,multi-GHz signal rates per 10×10 cm sub-panel are possible whilerejecting most ghost hits. A conservative fraction of that rate, 200 MHzper sub-panel, is well within the capability of a modern FPGA.

One embodiment uses FPGAs over ASICs because they are fast enough andhave the important advantage of design flexibility. This flexibility isideally suited to examine the temporal and positional information in thebit streams, and developing algorithms to associate, refine, and compactdata from nearby cells into an overall cluster or track segment. Forexample, an embodiment with a design having one processing section toassemble the primitives (time and strip) with a second section to docorrelations is flexible and offers significant data compaction (i.e.,no hit, no bandwidth used).

In one embodiment for a small device designed for extremely high-ratecounting, the electronics include “coincidence” logic. Hits are to berepresented in 2 dimensions (for example, 16 bits×2), plus a timecoordinate based on a synchronous 100 MHz clock and a 16 stage delayloop. The time coordinate can be represented as a 12 bit clock countwith a 4 bit sub-interval, giving a sub-nanosecond time stamp.Transmitting these 48 bits continuously at 500 MHz is a non-trivialtask. Therefore a simplified design would provide buffers withsufficient space to hold all 2 coordinate hits from multiple pixelfirings, and multiplex these hits to a single 1.6 GHz fiber. Output fromthis fiber is then directed into a computer based fiber receiver. Thehigh speed data flow from the sub-panel would be accepted until thefiber buffers reach a limit (typically ⅔ full). Later, when the computerhas reduced the buffer content below ⅓, the buffer will be enabled toreceive additional data.

Referring again to FIG. 5, the side view of ultra-thin window gaschamber enclosure 501 shows two sets of double foil windows 503 inaccordance with one embodiment. Although FIGS. 4, 5 and 6 identify theultra-thin gas chamber windows 403, 503 and 603, respectively, as foilwindows, in other embodiments windows 403 503 and 603 are coated polymerthin-film windows which includes metalized polymer films. In otherembodiments, the ultra-thin windows could be multilayered metal foilssuch as nickel, aluminum, copper, gold, etc., coated titanium foil, ortitanium, nickel, copper, gold, etc., coated aluminum foil, or any othermetal metal-foil combination including beryllium foil or alloy foilsbased on metals such as iron, cobalt, nickel or molybdenum, for examplestainless steel type foils. In most embodiments, the gas mixturecomponent composition between the double foil windows will be the sameas the gas mixture component composition inside the chamber and flowingthrough PPS panel 502 of FIG. 5, or through PPS multiple panels 602 ofFIG. 6. In one embodiment, the gas pressure between the double foilwindows is approximately the same as the gas pressure inside the chamberand flowing through PPS panel 502, or through PPS multiple panels 602.In other embodiments, the gas pressure between the double foil windowscan be either greater than, or less than, the gas pressure inside thechamber and flowing through the PPS panel or panels.

The gas chamber enclosures in FIGS. 4, 5 and 6 are shown and describedas allowing continuous gas flow, with each chamber having a set ofentrance and exit gas ports or gas control valves 405-406, 505-506 and605-606, respectively, to allow gas to flow in and flow out asillustrated by the arrows next to each valve. This style of gascontinuously flowing through the chamber is sometimes referred to as a“dynamic gas flow chamber.” However, in some embodiments the abovedescribed gas chambers could be operated as static gas-filled chambersrequiring only a single gas valve, especially if the chambers aredesigned and fabricated as ultrahigh vacuum (“UHV”) systems employingConflat-style CF-flanges with metal gaskets. Depending upon theultra-thin window material composition, the window aperture size (e.g.,inside diameter opening), the pressure differential (i.e., the pressuredifference between inside and outside the chamber), the number ofparallel ultra-thin windows (i.e., single vs. double vs. tripleultra-thin window configuration), whether or not the ultra-thin windowis backed up with a mechanical support structure or system, and whetheror not the ultra-thin window chamber is designed as a static or dynamicgas system, each individual ultra-thin window thickness can vary over avery wide range spanning, for example from about 1 to 200 microns.

In the case of a static-UHV chamber, which requires only a single valvefor both gas evacuation and gas fill, such as 405, 505 or 605, thesecond valve 406, 506 or 606 can be respectively eliminated. In oneembodiment of a static-UHV chamber, the ultra-thin chamber window can befabricated from metal foils such as titanium having a thickness fallingwithin at least a ten-fold thickness range, for example fromapproximately 0.0005″ to 0.005″. In another static-UHV chamberembodiment, a double ultra-thin metal foil layer window arrangementcould be employed, for example with each layer consisting of a 0.0003″to 0.003″ thickness of titanium foil.

For the thinnest windows, a dynamic gas flow-through chamberconstruction is favored as it can tolerate more gaseous diffusion andgas permeation through the ultra-thin window material because theinternal chamber gas is being constantly exchanged and replenished.However, for maximum simplicity and minimum cost in terms of not needingto stock and maintain an onsite gas storage and gasexchange/distribution system, a slightly thicker window material designis favored with a static-UHV chamber construction. In anotherembodiment, the ultra-thin chamber windows could also be fabricated astriple layer windows, being either a triple layer of metal foil or atriple layer of metalized polymer film, or some combination of each.Such an arrangement could be used in either a gas flow-through chamberor a static gas-filled chamber. The gas composition between the multipleultra-thin window layers would be approximately the same as that insidethe chamber and flowing through the PPS panel or panels.

Similarly, for a two or more layer ultra-thin window construction suchas in FIG. 5, the gas pressure in the volume between the foil windows503 could be the same, less than, or more than, the gas pressure insidethe chamber and flowing through the PPS panel or panels. In theembodiments based on a static gas-filled chamber design andconstruction, the chamber would periodically require occasional gasrefilling or recharging. For well-designed UHV chambers with foilwindows, gas contamination from outgassing, diffusion and permeation,could be at a slow enough rate such that the chamber is able tosatisfactorily maintain its gas-filled composition integrity for aperiod of months or possibly even years, depending upon the chamberconstruction and materials, and on the specific application performancerequirements.

Depending upon the difference between the chamber internal gas pressure,and the ambient external gas pressure, as well as the ultra-thin windowsurface area dimensions and thickness, in some embodiments the windowwill incorporate internal and/or external structural reinforcement(e.g., some type of wire or narrow grid wall patterned structuralreinforcement). In some embodiments, the gas mixture could contain ahalogen gas component such as Br₂, Cl₂ or F₂, in combination with aninert gas (e.g., He, Ne, Ar, Kr or Xe), or an inert gas mixture thatcould also include other relatively inert gas components such as: N₂,SF₆, CO₂, or fluorocarbons such as: CF₄, C₂F₆, C₃F₈, C₄F₈, C₄F₁₀, C₅F₁₂,etc. Other embodiments of gas mixtures could contain any combination ofthe above gas components.

The ultra-thin dielectric substrates as described herein can alsoinclude inorganic substrate materials that are crystalline in naturesuch as micas which are commercially available in thicknesses down toabout 5 microns. In one embodiment, the ultra-thin PPS panels asdescribed herein (e.g., in FIGS. 2, 4, 10, 12, 14 and 15) can befabricated using mica substrates that range in thickness from about 5 to20 microns.

Depending upon the application, the PPS panels as described herein(e.g., FIGS. 1-6, 8, 10, 12, 14 and 15) can suffer from charge buildupon the front and/or back dielectric substrate surfaces. Although chargescan accumulate on both the anode or cathode substrate surfaces, the mostserious charge buildup will tend to occur of negative charge on theanode substrate side. This charge buildup can degrade the PPS deviceperformance over time, because as the charge accumulates the netelectric field diminishes, which can adversely affect device efficiencyand long-term stability. In order to minimize or eliminate this problem,one option is to select a dielectric substrate with sufficiently lowsurface resistivity such that it can bleed off the charge as it isdeposited and accumulates on the dielectric surface. For example, in oneembodiment ESD-Alumina by CoorsTek (also marketed under the nameStatSafe™ Alumina) can serve as the panel substrate on the anode side.In another embodiment, ESD zirconia by CoorsTek, marketed under the nameCeraStat® Zirconia, can serve as the panel substrate on the anode side.However, if the surface resistivity is too low, then the PPS performancecould be compromised by the electrodes effectively shorting out. A moreuniversal approach is to coat the dielectric substrate with anappropriate thin-film layer having the desired surface resistivity.However, any such thin-film dielectric coating must also have therequisite physical, electrical and chemical properties so as not todegrade or react with the discharge gas or adversely interact with theplasma discharge process or with electrode operation.

A number of different coating materials and coating thicknesses can besuccessfully employed for the purpose of adjusting, altering,controlling and/or modifying the substrate surface resistivity. Examplesinclude a variety of pure, or mixed, or blended, or doped crystalline,or partially crystalline, or amorphous dielectric materials having therequired surface resistivity, including materials specifically processedto reduce their surface resistivity such as by being oxygen depleted.Such materials can include a select group of semiconductors, glasses andceramics, as well as various blends or mixtures or alloys or dopedblends of such materials. Some embodiments of such materials includethin-film coatings of electrostatic discharge-safe (i.e., ESD-safe)ceramics engineered to prevent charge accumulation and/or staticelectricity buildup, with typical thicknesses in the range of 0.01 to 10microns. Examples of a few such types of bulk ESD-safe materials includetrade name products such as: StatSafe™, ArcResist™, and CeraStat®. Forexample, three such materials include: StatSafe™ Alumina, StatSafe™Silicon Carbide, and CeraStat® Zirconia. In one such embodiment, athin-film coating of StatSafe Alumina, also known as ESD-Alumina orADC-92 (i.e., a specially processed, mixed ceramic containingapproximately 92% alumina), was successfully coated on both glass andceramic substrates. Lower surface resistivity materials than ESD-Aluminainclude: boron carbide (B₄C) as well as pure boron itself, pureintrinsic silicon (Si), silicon carbide (SiC), tungsten carbide (WC),etc.

Several embodiments are specifically illustrated and/or describedherein. However, it will be appreciated that modifications andvariations of the disclosed embodiments are covered by the aboveteachings and within the purview of the appended claims withoutdeparting from the spirit and intended scope of the invention.

What is claimed is:
 1. An ultra-thin radiation detector comprising: aradiation detector gas chamber having at least one ultra-thin chamberwindow; an ultra-thin first substrate contained within the gas chamber;a second substrate generally parallel to and coupled to the firstsubstrate and defining a gas gap between the first substrate and thesecond substrate; a discharge gas between the first and secondsubstrates and contained within the gas chamber, wherein the dischargegas is free to circulate within the gas chamber and between the firstand second substrates at a given gas pressure; at least one firstelectrode coupled to one of the substrates; at least one secondelectrode electrically coupled to the first electrode; a first impedancecoupled to the first electrode; a power supply coupled to at least oneof the electrodes; a first discharge event detector coupled to at leastone of the electrodes for detecting a gas discharge counting event inthe electrode; and a plurality of pixels defined by the electrodes, eachpixel capable of outputting a gas discharge counting event pulse uponinteraction with ionizing radiation.
 2. The radiation detector of claim1, wherein the gas chamber has a set of entrance and exit gas controlvalves to allow gas to continuously flow through the gas chamber.
 3. Theradiation detector of claim 1, wherein the gas chamber is constructed asan ultrahigh vacuum chamber and configured to operate as a staticgas-filled chamber requiring only a single gas valve.
 4. The radiationdetector of claim 3, wherein the ultrahigh vacuum chamber comprisesCF-flanges with metal gaskets.
 5. The radiation detector of claim 3,wherein the gas chamber has at least two separated, parallel, ultra-thinchamber windows located on a same chamber side.
 6. The radiationdetector of claim 3, wherein at least one of the ultra-thin chamberwindows comprises an ultra-thin metal foil.
 7. The radiation detector ofclaim 3, wherein the gas chamber has a second set of at least twoseparated, parallel, ultra-thin chamber windows, parallel to a first setof ultra-thin chamber windows and located on the opposite chamber side.8. The radiation detector of claim 6, wherein the at least one of theultra-thin foil chamber windows comprises a titanium foil windowmaterial.
 9. The radiation detector of claim 1, wherein the gas chamberhas at least two separated, parallel, ultra-thin chamber windows locatedon a same chamber side.
 10. The radiation detector of claim 9, whereinat least one of the ultra-thin chamber windows comprises an ultra-thinmetal foil.
 11. The radiation detector of claim 9, wherein at least oneof the ultra-thin chamber windows comprises a metalized ultra-thinpolymer film.
 12. The radiation detector of claim 9, wherein the gaschamber has a second set of at least two separated, parallel, ultra-thinchamber windows, parallel to a first set of ultra-thin chamber windowsand located on the opposite chamber side.
 13. The radiation detector ofclaim 10, wherein at least one of the ultra-thin foil chamber windowscomprises a titanium foil window material.
 14. The radiation detector ofclaim 1, wherein the discharge gas comprises at least one inert gascomponent.
 15. The radiation detector of claim 1, wherein the dischargegas comprises at least one fluorocarbon gas component.
 16. The radiationdetector of claim 1, wherein the discharge gas comprises at least onehalogen gas component.
 17. The radiation detector of claim 1, whereinthe discharge gas comprises at least one fluorocarbon and one halogengas component.
 18. The radiation detector of claim 14, wherein thedischarge gas comprises at least one fluorocarbon gas component.
 19. Theradiation detector of claim 18, wherein the discharge gas comprises atleast one halogen gas component.
 20. A method of detecting radiationcomprising: receiving radiation at an ultra-thin first substratecontained within a radiation detector gas chamber having at least oneultra-thin chamber window; creating at least one ion-pair in a gascontained within a gas gap between the first substrate and a secondsubstrate generally parallel to and coupled to the first substrate; andcausing a gas-discharge event at a pixel, each pixel site defined by ananode and cathode that, wherein the discharge event is isolated and eachpixel is capable of outputting a gas discharge counting event pulse uponinteraction with ionizing radiation.